Sense amplifier circuit and method for a dram

ABSTRACT

A sense amplifier of a DRAM includes, in series between two terminals of application of a supply voltage, at least one first transistor of a first channel type, and an amplification stage formed of two parallel branches each including a second transistor of the first channel type in series with a transistor of a second channel type. The gates of the transistors of a same branch are connected to the junction point of the transistors of the other branch. Each branch including at least one first additional transistor of the first channel type in parallel with at least each second transistor of the first channel type.

PRIORITY CLAIM

The present application is a Continuation of copending U.S. patentapplication Ser. No. 12/439,728, filed May 23, 2006; which applicationclaims the benefit of French Patent Application Serial No. 05/51336,filed May 23, 2005; all of the foregoing applications are incorporatedherein by reference in their entireties.

TECHNICAL FIELD

Embodiments of the present invention generally relate to DRAMs and, morespecifically, to a DRAM cell sense amplifier.

BACKGROUND

FIG. 1 shows a conventional example of a DRAM cell sense amplifier in aconfiguration with precharge to the ground. To simplify therepresentation, the actual memory cells have not been shown, and anexample of a memory architecture of the type to which embodiments of thepresent invention apply will be described in relation with FIG. 2.

Amplifier 1 comprises, between two terminals 2 and 3 of application ofpower supply voltages, respectively Vdd and GND (possibly via a switchnot shown), a P-channel MOS transistor MP1 and an amplification stageforming a flip-flop, formed of two parallel branches each comprising aP-channel MOS transistor MP2, respectively MP3, and an N-channel MOStransistor, respectively MN2 and MN3. The junctions 4 and 5 of therespective series associations of transistors MP2 and MN2, and oftransistors MP3 and MN3, are connected to so-called true or direct andcomplementary bit lines TBL and CBL of the memory cell column of thearray network to which amplifier 1 is assigned. Terminals 4 and 5 alsodefine respectively direct and complementary output terminals of senseamplifier 1. The gates of transistors MP2 and MN2 are connected to point5 while the gates of transistors MP3 and MN3 are connected to point 4.

At each read cycle, a single memory cell of the column connected toamplifier 1 is read from, said cell being addressed by selecting aso-called word line by means of a decoder, not shown.

FIG. 2 very schematically and partially shows an example of architectureof a DRAM of the type to which embodiments of the present inventionapply. For simplification, a single amplifier 1 (SAi) has been shown.Amplifier 1 is used to read bit lines CBLi and TBLi to which areconnected memory cells 10T(i,j), 10T(i,j+1) on line TBLi and cells10C(i,j) and 10C(i,j+1) on line CBLi. Each cell is formed of a selectiontransistor T and of a capacitor C between bit line CBLi or TBLi and abias potential Vp. The respective gates of transistors Tare connected toword lines WLj and WLj+1. In FIG. 2, cells 10C(i,j) and 10T(i,j+1) havebeen shown in dotted lines to illustrate the fact that, to read, forexample, from cell 10T(i,j) of line i and of row j, the amplifier SAiassigned to the lines of rank i performs the reading from line TBLiwhile line CBLi is used as a reference line.

To read from the cells of a DRAM, the bit lines need to be precharged ata given voltage.

A first category of DRAMs provides a precharge of the bit lines to halfthe supply voltage, Vdd/2.

A second category of DRAMs to which embodiments of the present inventionmore specifically apply provides a precharge of the bit lines to groundto make the reading faster.

In this case, for each bit line, at least one reference cell 20 is used.Each reference cell 20 is typically formed of a memory point formed of atransistor T and of a capacitor C between line CBLi or TBLi and voltageVp. The gates of transistors T are respectively connected to selectionlines RefWL1 and RefWL2. Further, the memory points of cells 20 areconnected by a transistor T′ to a line 21 of application of a referencevoltage Vref (for example, equal to Vdd/2) by being simultaneouslycontrolled by a signal PREF of precharge of the reference cells.

DRAMs using reference cells in a ground precharge configuration are knowin the art.

The use of reference cells in DRAMs increases the bulk of these memories(4 reference cells are further necessary in case of twisted bit lines inthe above-mentioned example).

Further, the use of reference cells requires control of additionaltransistors, which generates an undesirable consumption equivalent tothe reading from four cells instead of one.

Another disadvantage is that this lengthens the read cycles due to thetime required to balance the charges of the reference cells.

Also known are sense amplifiers having parallel connected branches oftransistors in series, in which junction points of the seriesassociations are not connected to bit lines, the bit lines beingconnected to gates of additional transistors connected in parallel withtransistors of the branches.

SUMMARY

Embodiments of the present invention overcome all or part of thedisadvantages of conventional DRAMs and of their reading.

Another embodiment of the present invention is, more specifically,providing a solution avoiding use of reference cells.

A further embodiment of the present invention provides improvedsensitivity of the sense amplifier.

According to one embodiment of the present invention, a sense amplifierof a DRAM includes in series between two terminals of application of asupply voltage at least one first transistor of a first channel type andan amplification stage formed of two parallel branches each including asecond transistor of the first channel type in series with a transistorof a second channel type. The gates of the transistors of a same branchare connected to the junction of the transistors of the other branch.Each branch includes at least one first additional transistor of thefirst channel type in parallel with at least each second transistor ofthe first channel type.

According to an embodiment of the present invention, each firstadditional transistor is in parallel exclusively with the secondtransistor of the first channel type of the involved branch.

According to an embodiment of the present invention, each firstadditional transistor is in series with a second additional transistorof the same type between a first terminal of application of the supplyvoltage and said junction of the involved branch.

According to an embodiment of the present invention, one of said firstadditional transistors is turned on before turning-on of the firsttransistor of the first channel type.

A further embodiment of the present invention is a method forcontrolling a DRAM sense amplifier, which includes the turning-on of thefirst additional transistor which is connected in parallel with theamplification branch opposite to that containing the read memory cells.

Another embodiment of the present invention is a DRAM of the typecomprising an array network of cells connected to direct andcomplementary columns of bit lines and to word lines, and senseamplifiers.

According to an embodiment of the present invention, the memory has noreference cell precharged to a fixed voltage.

The foregoing features and advantages of the present invention will bediscussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2, previously described, are intended to show the state ofthe art and the problem to solve;

FIG. 3 shows a DRAM cell sense amplifier according to an embodiment ofthe present invention

FIGS. 4A and 4B illustrate the operation of the amplifier of FIG. 3,respectively for the reading of a state 0 and of a state 1; and

FIG. 5 is a schematic of a DRAM cell sense amplifier according to analternative embodiment of the present invention.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

Same elements have been designated with same reference numerals in thedifferent drawings. For clarity, only those elements which are necessaryto the understanding of embodiments of the present invention have beenshown in the drawings and will be described hereafter. In particular,the control circuits of the different memory elements have not beendetailed, with embodiments of the present invention being compatiblewith conventional circuits and especially line and column decoders.Further, the generation and the synchronization of the control signalsof the sense amplifier have not been detailed, with embodiments of thepresent invention being here again compatible with tools generally usedto generate such signals.

FIG. 3 shows a DRAM cell sense amplifier 30 according to an embodimentof the present invention.

As previously discussed, a P-channel transistor MP1 for controllingamplifier 30 connects a terminal 2 of application of a voltage Vdd to astage forming a flip-flop formed of two parallel branches, eachcomprising a P-channel MOS transistor MP2, respectively, MP3, in serieswith an N-channel transistor MN2, respectively MN3. The respectivesources of transistors MN2 and MN3 are connected to a terminal 3 ofapplication of a ground voltage GND while the respective sources oftransistors MP2 and MP3 are connected to the drain of transistor MP1.The junction points of transistors MP2, MN2, respectively, MP3 and MN3,define input/output terminals 4 and 5 of the sense amplifier intended tobe respectively connected to a direct bit line TBL and to acomplementary bit line CBL. The gates of transistors MP2 and MN2 areconnected together to point 5 while the gates of transistors MP3 and MN3are connected together to point 4 of the other branch. Points 4 and 5define the output terminals of the amplifier.

According to this embodiment of the present invention, two additionalP-channel transistors MP4 and MP5 are connected in parallel respectivelyon transistors MP2 and MP3. Transistors MP4 and MP5 are controlled bysignals SENSE CB and SENSE TB. Transistors MP4 and MP5 have the functionof dynamically biasing amplifier 30 during read cycles.

The memory architecture resembles that illustrated in FIG. 2, butwithout reference cells 20, which are now no longer necessary.

FIG. 4A illustrates the operation of amplifier 30 of FIG. 3 for thereading of a state 0 from line TBL.

It is assumed that the cell to be read from has been addressed by theword line before a time t10 so that no signal has been developed on lineTBL, the read cell being at state 0. At time t10, signal SENSE TB isswitched to state 0 to turn on transistor MP5 (transistor MP4 remainingoff). At a subsequent time t11, signal SENSE of amplifier 30 is switchedto the low state to turn on transistor MP1. The voltage of the two linesTBL and CBL starts increasing since both P-channel MOS transistors MP2and MP3 are in the on state. Further, an additional current flowsthrough transistor MP5, which causes an imbalance in the amplifier tothe advantage of line CBL, which increases faster. This voltagedifference is amplified by the flip-flop circuit and, at a time t12,line CBL is at the high state while line TBL is at the low state.Biasing transistor MP5 may be blocked from this time on by a stateswitching of signal SENSE TB.

FIG. 4B illustrates the reading of a state 1 from line TBL. In thisconfiguration, line TBL is not at state 0 but at a higher state(typically, a few hundreds of millivolts) before time t10, due to theopening of the memory point to be read by the switching of thecorresponding word line. At time t11 where transistor MP5 is turned onby the switching to the low state of signal SENSE TB, the voltage ofline CBL starts increasing. An additional current flows throughtransistor MP5. However, due to initial interval DV between the voltagesof lines TBL and CBL, this current is not sufficient to switch line CBLto level Vdd and the obtained amplification results in an increase inthe voltage of node 4 up to level Vdd, line CBL returning to 0.

Interval DV between the initial levels of lines TBL and CBL conditionsthe maximum width-to-length ratio (W/L) of transistors MP5 and MP4. Thisratio must then not be too high to avoid, on turning-on of transistorMP1, for a reading of a state 1 to result in a switching of theamplification cell.

Time t10 when transistor MP5 is turned on (or MP4 if the reading isperformed from line CBL) is of no importance, provided that it isperformed before the amplification phase (before time t11).

Transistors MP2 and MP3 are those which must conduct, as in aconventional amplifier, the read current. They must further be fast ascompared with transistors MP4 and MP5, which do not have this need.

The sizing of transistors MP2 and MP3 calls for a compromise since theyare ideally desired to be fast (requiring a large width (W) to length(L) ratio), small (requiring small dimensions Wand L) and matched(requiring a large product W*L).

Since transistors MP4 and MP5 do not have to be fast, the two otherrequirements (small and matched) result in dimension criteria compatiblewith each other.

According to an alternative embodiment shown in FIG. 5, the transistorsMP4 and MP5 are each coupled in series with a second transistor MP6 andMP7, respectively, connecting them to supply line Vdd 2, which are thencontrolled by the signal SENSE. Such transistors MP6 and MP7 in thisexample have a P channel and generate a slight increase in the amplifiersurface area.

In the illustrated example, the bulks of transistors MP2, MP3, MP4, andMP5 are brought to level Vdd, the bulks of transistors MN2, MN3, and MP1being connected to their respective sources.

An advantage of embodiments of the present invention is that they enablesuppression of the reference cells in the DRAM lines.

Another advantage of embodiments of the present invention is that it isnow no longer necessary to provide voltage level generators for thesereferences.

Another advantage of embodiments of the present invention is that theydo not alter the amplifier performances while suppressing the referencecells.

Of course, embodiments of the present invention are likely to havevarious alterations, modifications, and improvements which will readilyoccur to those skilled in the art. In particular, the respectivedimensions to be given to the different transistors of the amplifier arewithin the abilities of those skilled in the art based on the functionalindications given hereabove and on the application.

Further, the adaptation of the control signals of the differenttransistors is within the abilities of those skilled in the art.

Moreover, if the connections of the different bulks of the transistorshave been specified in the discussed embodiments, these are examplesonly and these connections may be adapted according to the application.

Finally, although an embodiment of the present invention has beendescribed in relation with an application where the precharge is toground, it also applies to a precharge to the positive supply level(Vdd) by inverting the transistor types and with an N-channel transistorcontrolled by signal SENSE connecting the differential amplificationstate to ground.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

A DRAM including sense amplifiers according to an embodiment of thepresent invention may be contained in a wide range of different types ofelectronic devices, such as in computer systems, cellular telephones,personal digital assistants, and so on.

1. A sense amplifier of a DRAM comprising, in series between twoterminals of application of a power supply voltage: at least one firsttransistor of a first channel type; and an amplification stage formed oftwo parallel branches each comprising a second transistor of the firstchannel type in series with a transistor of a second channel type, thegates of the transistors of a same branch being connected to thejunction of the transistors of the other branch, each junction pointdefining an input/output terminal of the sense amplifier intended to beconnected to a bit line, wherein each branch comprises at least onefirst additional transistor of the first channel type connected inparallel with each second transistor of the first channel type, eachfirst additional transistor being connected in parallel with each secondtransistor during an entire read cycle of the DRAM. 2-17. (canceled) 18.A method of sensing data stored in a memory cell coupled to a firstcomplementary bit line, a sense amplifier being coupled between thefirst complementary bit line and a second complementary bit line and themethod comprising: accessing the memory cell from which data is sensedto thereby develop a voltage on the first complementary bit line, thevoltage corresponding to the data stored in the cell; supplying adynamic biasing current to the second complementary bit line; activatingthe sense amplifier; driving a voltage on the first complementary bitline to a first voltage level and driving a voltage on the secondcomplementary bit line to a second voltage level through the senseamplifier; and after the operation of activating, terminating thesupplying of the dynamic biasing current to the second complementary bitline.
 19. The method of claim 18 wherein the dynamic biasing current issupplied through a first channel type transistor in response to a sensebit line signal. 20-22. (canceled)
 23. The method of claim 18 whereinthe first voltage level comprises a supply voltage and the secondvoltage level is the supply voltage divided by two.
 24. The amplifier ofclaim 1, wherein each first additional transistor is in parallelexclusively with the second transistor of the first channel type of theinvolved branch.
 25. The amplifier of claim 1, wherein each firstadditional transistor is in series with a second additional transistorof the same type between a first terminal of application of the supplyvoltage and said junction of the involved branch.
 26. The amplifier ofclaim 1, wherein one of said first additional transistors is turned onbefore turning-on the first transistor of the first channel type.
 27. Asense amplifier comprising: a first inverter and a second inverter, theinverters being cross-coupled between a first bit line node and a secondbit line node, and each inverter including a first channel typetransistor and a second channel type transistor coupled in seriesbetween a first node adapted to receive a first voltage and a secondnode adapted to receive a second voltage, with the first bit line nodebeing defined at the interconnection between the transistors in thefirst inverter and the second bit line node being defined at theinterconnection between the transistors in the second inverter; a firstadditional transistor coupled in parallel with the first channel typetransistor of the first inverter and adapted to receive a sense firstbit line signal; a second additional transistor coupled in parallel withthe first channel type transistor of the second inverter and adapted toreceive a sense second bit line signal; and wherein at least one of thefirst and second additional transistors is activated prior to the firstvoltage being supplied to the first node.
 28. The sense amplifier ofclaim 27 wherein the first channel type transistor comprises a PMOStransistor and the second channel type transistor comprises an NMOStransistor, and wherein the first and second additional transistors arePMOS transistors.
 29. The sense amplifier of claim 27 wherein the firstchannel type transistors in the first and second inverters are matchedtransistors and the first and second additional transistors are matchedtransistors, and wherein each of the first channel type transistors hasa channel width that is greater than a channel width of the first andsecond additional transistors.
 30. The sense amplifier of claim 27wherein the first voltage comprises a supply voltage and the secondvoltage comprises a ground voltage.
 31. The sense amplifier of claim 27further comprising a third additional transistor coupled between thefirst node and a node adapted to receive the first voltage, and whereinthe third additional transistor is operable to be activated after atleast one of the first and second additional transistors is activated.32. The method of claim 18 wherein activating the sense amplifiercomprises turning ON a transistor coupled between the sense amplifierand a node adapted to receive a supply voltage.